1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, and more specifically relates to an optical scanning device that scans a surface to be scanned with light and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
An image forming apparatus such as an optical printer, a digital copying machine, and an optical plotter includes an optical scanning device that scans a surface to be scanned with light modulated by image information to form a latent image on the surface based on the image information.
In the optical scanning device, a light beam emitted from a light source is deflected by a polygon mirror that rotates, for example (see Japanese Patent Application Laid-open No. 2008-180972, Japanese Patent Application Laid-open No. 2002-267990, Japanese Patent No. 4007111, and Japanese Patent No. 3192271, for example).
Because a polygon mirror rotates at a speed as high as several ten thousands in revolutions per minute (rpm), continuous rotation heats a drive mechanism (especially a bearing unit included in the drive mechanism).
In general, a polygon mirror is provided to a rotor. Heat generated at the bearing unit supporting the rotating shaft of the rotor may be conducted to the polygon mirror via the rotating shaft, thereby raising the temperature of the polygon mirror by several tens of degrees in centigrade. Accordingly, the rotation of the polygon mirror causes a flow of hot air that blows to a scanning lens and the like.
Recently, the number of scanning lenses made by resin mold has been increasing due to cost reduction. A loss of a uniform temperature distribution across a resinous scanning lens due to the flow of hot air blown by the polygon mirror may cause a change in refractive index, thermal deformation, a change in posture, and the like, whereby the performance of image formation is deteriorated.
With the downsizing of an optical scanning device, the diameter of a polygon mirror used in the optical scanning device is also decreased, whereby reducing the number of mirror facets mounted on the polygon mirror to further require a higher rotation speed, resulting in disadvantage by an increase of heat generation.
Conventionally, an optical scanning device has been provided with a cooling fan that forcibly causes a flow of air, having been taken in from outside the optical scanning device, to blow a polygon mirror and the optical scanning device, whereby the polygon mirror and the optical scanning device have been cooled. However, an increase in the speed of an image forming process in recent years has increased an amount of heat generation to cause a cooling fan to be driven to rotate even faster. A fast rotation of the cooling fan has caused an inconvenience with an increase of electricity consumption and noise increase.